This invention relates to optical transmission filters and, in particular, to methods of making improved transmission filters and the resulting improved products and improved equipment that they permit. The invention is particularly useful for filters, such as ultraviolet light filters, for which completely transparent filter materials are not available.
Optical transmission filters are useful in a wide variety of applications including spectroscopy and fluorescence microscopy. Filters are used in these applications to block unwanted light that would otherwise manifest as spurious light that could swamp the signals to be detected or distort the images to be seen.
Optical transmission filters typically transmit a desired range of wavelengths (referred to as a transmission band) and block wavelengths outside the transmission band (out-of-band wavelengths). Ideally they would transmit all light within a desired band and block all light outside the band. In reality, in-band transmission incurs some attenuation and out-of-band blocking is incomplete. Moreover, the spectral extent of blocking is limited, i.e. blocking may substantially diminish for light of wavelengths not far removed from the transmission band.
Referring to the drawings, FIG. 1A schematically illustrates the spectral transmission of an ideal optical transmission filter. An ideal filter would transmit all light having wavelengths within a band between a low wavelength λL and a high wavelength λH (the Transmission, T, is equal to 1). An ideal filter would block all light outside the band (T=0). λL and λH are the wavelengths at which the filter transitions between blocking and transmitting.
Real filters invariably block a small portion of the light to transmitted (T<1) and transmit a small portion of the light to be blocked (T>0). Moreover, the blocking may become less effective for wavelengths spectrally away from the transmission band. These properties are schematically illustrated in FIG. 1B showing the effect of a filter with less than perfect transmission, finite transition regions, less than perfect blocking, and out-of-band transmission. The proportion of light transmitted, the steepness of the transition lines and the extent of the blocking are important parameters in many applications.
Transmission filters are particularly important in optical measurement and analysis systems. Some such systems, e.g. fluorescence systems, use light of one wavelength to excite a sample of material and then measure or view an optical response of the excited sample at another wavelength. The excitation light is delivered to the sample by an excitation light path, and the optical response of the sample is delivered to the eye or measuring instrument by a collection path. Transmission filters between the source and the sample can be used to block spurious light from the excitation path. The steeper the filter transition lines, the more effectively spurious signals are blocked. The lower the transmission loss, the more light from the desired excitation band reaches the sample. Moreover, if the optical response being measured differs considerably in wavelength from the excitation light, the transmission filter needs extended out-of-band blocking to prevent transmission of spurious light that can scatter into the collection path.
UV fluorescence spectroscopy is based on the fact that when some materials are excited by ultraviolet light (light that is composed of wavelengths too short to be visible) they respond by the emission of near-UV and/or visible light (“fluorescent light”). In such apparatus it is important that the UV excitation path not transmit visible light that can also be transmitted as a spurious signal through the collection path.
FIG. 2 is a simplified schematic diagram of a UV probe 20 designed to excite a sample 21 by UV light and to collect visible fluorescent light from the sample. In essence, the probe comprises a UV source 22, an excitation path 23 for transmitting the UV light to the sample 21 and a collection path 24 for transmitting the fluorescent response light from the sample 21 to a detector 25. The excitation path 23 for UV light ideally transmits only UV light that will excite specific materials (“markers”) in the sample. In reality, UV light sources may include or can generate spurious visible light by a number of mechanisms. The spurious visible light can scatter off the sample 21 into the collection path 24 to the detector.
Absorption spectroscopy is another optical analysis technique used in identifying materials and quantifying concentrations. In absorption spectroscopy light of one or more discrete bandwidths is transmitted through a first path and through a sample. Light from the illuminated sample is transmitted through a collection path to a detector that can measure the amount of light the sample absorbed. The amount of absorption provides information regarding the identity of unknown materials or the concentration of known materials. Spurious light through the first path can provide incorrect or inaccurate results.
It should be clear that the steeper the filter slope at the transition wavelengths λL, λH, the greater the amount of unwanted light that can be filtered out, avoiding spurious results. The greater the in-band transmission of the filter, the greater the input of desired light. And the greater the extent of out-of-band blocking, the less spurious light at the output to interfere with measurement or viewing.
Systems using UV excitation light, particularly UV bands in the wavelength range 230 to 320 nanometers, are particularly useful. The term “bands within the wavelength range” as used herein is intended to include smaller wavelength bands included within the range, e.g. 250-270 nanometers, as well as bands that encompass the range, e.g. 220-325 nanometers. Bands within the 230-320 wavelength range have attained prominence for use in biomedical applications as diverse as drug discovery, genomics and proteomics, immunology, chemical process tracing and threat biodetection. UV bands in this range are highly useful in the fluorescent detection of nucleotides, proteins and enzymatic molecules.
Unfortunately existing filters and equipment leave much to be desired for bands within the 230 to 320 nanometer range. Few commercial filters are available in the range, and none provide the combination of high transmission, steep edge slopes and deep, extended out-of-band blocking needed for high performance detection and measurement.
Typical commercial filters are metal-dielectric filters and soft-coating thin film filters. Metal-dielectric filters provide relatively low transmission: typically only 10-30%. The low-transmission in such filters is inherent because transmission performance is inversely related with achievable blocking.
“Soft-coating” thin-film filters, as well as those that achieve partial blocking using colored absorbing glasses, are not suitable for high intensity light sources in the 230-320 nanometer range. In the UV range, soft-coating materials suffer from severe reliability, durability and spectral stability issues. Even moderate amounts of illumination by high intensity UV sources can cause soft coatings and colored substrates to “burn out”, solarize or photodarken.
A new approach to making highly discriminating optical filters and the resulting improved products are disclosed in U.S. Pat. No. 7,068,430 which is incorporated herein by reference. This approach offers considerable promise for application to filters for the visible range where essentially transparent materials are available. The present invention is an extension and modification of the '430 method to produce advantageous filters for shorter wavelengths where completely transparent materials are not available.